Various hydrophilic graft polymers obtained by graft-polymerization of unsaturated monomer component onto the main chain consisting of polyalkylene oxide have hitherto been developed, and have been used for various applications such as scale inhibitor, water treating agent, detergent builder, dispersant for poorly water soluble inorganic or organic materials (dispersant for inorganic pigments, dispersant for cement-water slurry), textile treating agent (for example, see U.S. Pat. Nos. 4,528,334; 4,612,352; EP-A-0639592; U.S. Pat. No. 5,952,432).
The above hydrophilic graft polymer is produced by graft polymerization accomplished by adding an unsaturated monomer such as acrylic acid, together with polymerization initiator and the like to a compound of polyalkylene oxide, and by heating them. In this case, there has been a problem that not a little amount of the unsaturated monomer component used as a raw material in the polymerizing reaction product after preparing the polymer is remaining. This is unfavorable with the view of productivity and performance of the polymer. Also, in particular, for human body contacting uses, reduction of toxicity and irritation due to residual monomer is required as low as possible. In order to solve such a problem, a hydrophilic graft polymer obtained by graft polymerizing a mono ethylenic unsaturated monocarboxylic acid and a mono ethylenic unsaturated dicarboxylic acid onto a polyether compound is disclosed in JP-A-2001-226441. Amount of the residual mono ethylenic unsaturated monocarboxylic acid in the polymer described in JP-A-2001-226441 is 900 ppm or less, and consequently, amount of residual monomer is relatively low compared with previous types of these graft polymers.
Also, the above hydrophilic graft polymer may not be good in temporal storage stability in relation to residual monomer. As a method to solve this problem, for example, in JP-A-11-279220, the production method of the hydrophilic graft copolymer containing the step performing the forced hydrolysis of the hydrophilic graft copolymer is disclosed.